Lumber sheathed diaphragm

[red22]

The project involves the renovation of an 100 year old building. Clay brick exterior bearing walls, steel frame interior supporting timber joists. Floor diaphragm is lumber sheathed (perpendicular to joists) with hardwood over top of planks (perpendicular to lumber sheathing).

All interior gypsum block/lath and plaster walls are being removed. We are providing new LMF shear walls to replace existing walls, although not in the same locations. IBC 2006 does not allow an increase in seismic force in any existing structural element of more than 10% without the entire lateral system meeting the requirements of ASCE 7 for new structures. The new locations of the shear walls will increase the force in the horizontal diaphragm by more than 10%. NDS and FEMA/NEHRP have values for lumber sheathed diaphragms, but I could not find any values or discussion of this type of diaphragm in IBC. Am I stuck having to add plywood to every floor, or locate shear walls to maintain the same sub-diaphragm aspect ratios?

 

[DonPhillips]

Since NDS is a referenced standard within IBC, you can use the NDS values to comply. This is covered in Chapter 1 of IBC under alternative methods.

Don Phillips

http://worthingtonengineering.com

 

[bylar]

Try IBC 2306.3.4

 

[red22]

Thanks. Unfortunately 2306.3.4 only references diagonally sheathed diaphragms.

Don, under the alternative methods section, 104.11, it would appear that the "alternative method" is based on the building officials opinion. So I guess, depending on the building official this may or may not be an accepted method.

 

[RARWOOD]

If you are moving the shear walls I would recommend adding plywood sheathing. I probably would do it even if I was not moving the shear walls.

Tranverse lumber sheathed diaphragms have very low shear values and usually large deflections. Another advantage of adding the plywood, is that you have well established shear values so when you are done you know you have a very reliable diaphragm.

 

[red22]

Ideally, I would like to add plywood to every floor, but of course cost is an issue. Just trying to determine the most cost effective, code compliant and safe design.

If the only sheathing were horizontal planks, the answer would be easy (NDS allowable shear = 100 plf) due to the very limited allowable shear value. However there is hardwood flooring on top of the planks spanning perpendicular to the planks. FEMA 356 gives values for this type of diaphragm (400 plf +/-) that I could live with, probably just using some additional nailing and chord reinforcing.

 

[DonPhillips]

Very true, Red, and may be worth a call to the CBO to get a decision before you apply for a permit. When I do a plan review, if I see a seal on the design, that design professional takes the responsibility and I do not get into the detail on how you got there. If I see something unusualy, I may call to understand what I am seeing, but seldom go beyond the phone call.

From a design professional liability prespective, if you are following the code, I think you would be on safe ground regarding neglience claims.


Don Phillips

http://worthingtonengineering.com